Method and means for determining fatigue damage and surface stress

ABSTRACT

The test specimen is mounted on a movable table, ultrasonic surface waves are caused to travel across the specimen by a transducer fed from a CW signal source, a laser beam is passed through a beam splitter, one portion being reflected onto a photo-detector measuring signal amplitude, an output signal is compared with the output of the CW signal and the CW signal shifted in phase by 90*, these outputs measure ultrasonic phase difference, at points along the specimen; the second portion of laser beam is reflected from a mirror attached to the specimen table and back to a screen where an interference pattern is formed as the table is moved, photo detectors sense the changes in interference pattern and determine specimen displacement.

XR 39854 @325 Coate I541 METHOD AND MEANS FOR DETERMINING FATIGUE DAMAGEAND SURFACE STRESS i151 Inventor: For'rest M. Coate; Hawthorne, Calif.

[73 Assignee: The United States of America as a represented by theSecretary of the l V Air Force, Washington, 'D.C. 122 Filed: July i3,191: 21 Appl. No; 319,029 c|.;;i ;'II.'.'.1. 13/613, 73/88 A" 511 rm.cl. com 29/04 [581 Field of Search 73167.5 R. 67.6;7ll3,. a a 1 73/88 A,an

s61 References Cited UNITEDSTATES PATENTS 3.47|.23a 10/1969;Hawke........ 73111.3 'ux v3.145.812 mm Korpel.. 13/o1.s R'

OTHER PUBLICATEONS R. L. Whitman et al.. Probing of Acoustic SurfacePrimaryEtariwiner-Richard C. Queisser Assistant Examiner-John P.Beauchamp 'Anomey, Agent, or Firm-Ha y Herbefl. In;-

Henry S Miller f The test specimen is mounted on a movable table. ulltrasonic surface waves arccaused to travel acrossthe 5 specimen by atransducer fed from a CW signal 1 source, a laser beam is passed througha beam splitter;

one portion being 'reflected onto a photo-detector I v measuring signalamplitude, an output signal is compared with the output of the CW signalandthe CW signal shifted-in phase by 90, these outputs measureultrasonic phase difference, at points along the specimen; the secondportion of .laser beam is reflected from a mirror attached to thespecimen table and back to a screen where an interference pattern isformed as the table is moved, photo detectors sense the changes ininterference pattern and determine specimen displacement. 4 r

' 5 Claims,'l Draining Figure r a y ya 2; N

L DIG/79L Y 71907 G Cawvrtt urinal zo 24 V alse I flan/r0)! 3541:170 J3: 3573/7! 15120 Deva-r42 8 wit u a ms: 9,. Jam 0' t. f ave/n44 3354325{1451' nee-17, 1974 METHOD AND Means FOR DETERMINING I amour: DAMAGE ANDSURFACE srnsss BACKGROUND or THE INVENTION? means for determining earlyfatigue damage and surf It is therefore anobject of the invention toprovide 7 This invention relates generally to a method and 5 face stressin metals and more particularly to such a de-- termination through theprecise measurement of ultrasonic surface wave velocity.

With the increased utilization of exotic metals and the increased use ofcommon metals to their maximum capability in aircraft, space flightparaphernalia and undersea exploration, the need for the earlydetermination of fatigue damage and surface stress becomes 'i moreurgent than in the past.

' Early fatigue damage appears first at the surface of the metal and canbest be detected ultrasonically by using the highest practical surfacewave frequency which travels in very close proximity to the metalsurface. in order ,to detect early damage, where the signs tigue'.

are not readily apparent. the .velocity and attenuation a new andimproved method and means for determining fatigue damage and surfacestress in metals.

It is another object of the invention to provide a new and improvedmethod of inspecting metal surfaces that is more accurate than anyhitherto known; Y

It is a further object of the invention to provide a new and improvedmeans for testing metals that is more effcient than similar devices ofthe prior art. g j

It is still another object of the invention to provide a new andimproved test means that may be easily operated.

rate must be measured with extreme accuracy if they are to reveal thesevery small changes associated with the early stages of fatigue damage. VKnown methods and means for detecting early damage have pro en less thansatisfactory. Known devices are cumbersome, introduce stress ordeformation into the specimen. by weight or contact pressure frommeasuring equipment, and, in general, fail to provide the extremeaccuracy needed to detect signs of metal fa- SUMMARY OF THE INVENTIONThe invention measures three parameters in order to successfully utilizeultrasonic waves in detecting early damage in metals. The amplitude ofthe ultrasonic surface wave is measured at selected locations on thesurface of a test specimen. Displacement is measured very accurately asthe specimen is moved laterally between The attenuation rate canbedetermined by knowing the amplitudes. of the surface wave at two pointsa,

it is still a further object of the invention to provide a new andimproved test means for metals that requires Y no physical connectionbetween the measuring system and the tested metal. I I

It is another object of the invention to provide a new and improvedmeans for inspecting metal surfaces that will not introduce stress ordeformation errors into the tested metal. v f lt is another object ofthe invention to provide a new and improved method of testing metalsurfaces that is fast, accurate and more reliable than any hithertoknown.

It is another object of the invention to providemeans for deten'niningfatigue damage and surface stress which is economical to produceand-utilizes convenv tional, currently available components that lendthemselves to standard mass production manufacuturing techniques. J

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiment in the accompanying drawings. orzscmmonor THE DRAWING I The F IGURE is a schematic representation of invention.

test positions. As the specimen travels from one posi- DESCRIPTION-OFrm-i PREFERRED I EMBODlMENT r The invention relates to the precisemeasurement of known distance apart along the path of the signal.Simiand frequency.

- The invention places a test specimen on a movable test stand or.table. Ultrasonic waves are emitted across f the surface by a surfacewave transponder fed from a 1 'CW ultrasonic signal source. A lower beamdirected 'toward the surface is passed through a beam splitter qualityof the metal.

where part of the beam is reflected from the surface to trphoto-detector. The remainder of the beam is reflected through a mirrorsystem to provide an interference pattern on 'a screen which detectsmovementof the table or test stand. The portion of the beam reflectedfrom the surface is sent to a photo-detector where signal amplitude ismeasured and the output is Compared with the original CW signal and theoriginal signal phase shifted by The phase shift is detected andrecorded and provides anlindication of the surface three parameterswhereby fatigue damage and surface stress in metals may be detectedusing ultrasonic meth-.'

ods. These three parameters include signal amplitude,

specimen displacement and ultrasonic phase difference.

Referring now to the FIGURE, the invention consists of a longitudinalmovable specimen Table 10 with appropriate means for mounting a specimenl2. -An ultra sonic CW signal source 14 is connected to a surface wavetransducer 16 positioned onthe specimen l2. Connected to the said sourceis a phase. splitter 18 for shifting the phgse of the CW signalsource by90.-

Phase sensitive detectors 20 and 22compare the phase of the signals fromthe phasesplitter 18 with the signal from the photo detector 24'. Theoutput of the detectors is sent to the digital'couter 24 which-indicatesfull wavelengths .with digital accuracy while partial wave-1 lengths areindicatedby the dial 2 6 attached to the.

shaft of two phase synchro receiver 28.

The measuring system consists of a laser 30 radiating through thebeamsplitting mirror 32 to the test speci- V men 12 at a point 34. Theinitial laser beam 33 passes through the grating 36 as does thereflected beam on its way to the detector 24. A means for providing anscreen 44.

amplitude readout from the photo detector is shown at 38 and will bediscussed hereinafter.

The remainder of the laser beam 38 is reflected through another beamsplitting device 40 where one portion of the beam is reflected from amirror 42 attached to the table and back to a screen 44. The remainderof the beam is reflected from a stationary mirror 46 to the screen 44.

' .A pair of photosensors and 50 sense the pattern on the screen 44 andsend a signal to the counter 52 e where table movement is measured.

The operation of the inventionfollows thusly; ultrasonic surface waveamplitude is measured by the angular displacement of the reflected laserbeam as the surface waves pass through the test point. in the F lGURE,the laser. beam travels first through a half silvered mirror or beamsplitter 32 which diverts part of the light for use in the specimendisplacement measuring system. The remaining light passes through anoptical grating 36 composed of alternate clear and opaque line pairsspaced approximately at the expected wavelength the screen 44. if thescreen is at a very slight angle or the light beams are not preciselyparallel, interference fringes will appear on the screen. The fringepattern will move one complete fringe width on the screen for eachdisplacement of the table length of the light.

Photodetectors 48 and 50 are used to detect the light travel at twolocations on the fringe pattern that are approximately 90 out of phase.This two phase information is fed to a digital indicator capable ofsensing the V direction of motion. The digital indicator will indicatedisplacement of the specimen table relative to an initial positionwithin one half wavelength of the laser light.

. Phase difference between the ultrasonic signals at 11 two test pointson a specimen -is determined by measurf. ing the phase shift or phaseslip relative to the ultra-- sonic frequency source while moving thedetector systern from one test point to another. The method of r theinterferometer but of the ultrasonic surface wave. The frequency of thenal source 14 is fed to both the surface wave transducer 7 I ameter andthe angle of reflection will be equal to the angle of incidence to theplane of the specimen. As the wave progresses a quarter wavelength thelight will strike the downward slope of each wave and will be reflectedat a small angle to the left as seen in the FIS- URE.

When the wave has moved another quarter wavemeasuring this phase changeis similar to that used in means. The signal from the continuous waveultrasonic sig- 16 and to the electrical phase splitter 18. The phasesplitterproduces two reference signals differing in phase by ninetydegrees. The transducer creates a train of surface waves'which travelacross the specimen, past the test point illuminated by the laser beam33. The

photo detector 24, used for amplitude, measurements i' also providesphase information to both of the phase sensitive detectors 20 and 22.Because the reference 7 phases are in quadrature, the outputs of thedetectors r are also in quadrature. As the path length of the surfacewave between transducer and test point changes one wavelength, the twophase signal to the phase shift digi length the trough will reflect thelight at the same angle as the crest. At the third quarter wavelengthposition. the reflection will'be to the right as seen in the FIG URE.The grating 36 is positioned in a manner so that approximately half ofeach reflected plane or slice of light will pass through the clearbands. then the energy striking the photodetector will be approximatelyproportional to the angle of reflection for small angles. This lightenergy will be modulated sinusoidally at the frequency of the ultrasonicsurface wave, with ans-1mplitude proportional to the amplitude of thesurface wave displacement. An AC voltmeter or oscilloscope (38) may beused to compare amplitudes at various test points on the specimen andthus provide an indication of attenutation.

Displacement or travel of the specimen table between test locations ismeasured with an interferometer system. A portion of the laser light(38) is diverted to a mirror system where it is divided and follows twopaths. The reference beam passes downward through a beam splitter40, isreflected by the lower mirror 46 and is reflected from the beam splittermirror to a ceiver 28.

tal indicator 24 rotates through one complete cycle- The indicatordisplays full wavelengths with digital accuracy. Partial wavelengths areindicated by the dial 26, attached to the shift of a two phase synchrore- It should be understood, of course, that the foregoing disclosurerelates to only a preferred embodiment of the invention and thatnumerous modifications or alterations may be made therein withoutdeparting from the spirit and scope of the invention setforth in the ap-Q pended claims.

lclaim:' l. A method for determining early fatigue damage and surfacestress in metals comprising the steps of:

transmitting an ultrasonic wave across a surface to be" examined;directing a beam of coherent radiation at a point on the said surface,through the ultrasonic wave;

detecting the reflected coherent radiation; moving the. surface in aknown manner; measuring the change in amplitude of the reflectedradiation, and comparing the phase relationship of the reflectedradiation to the ultrasonic wave whereby variations in saidamplitude andphase will be indicative of fatigue damage and surface stress.

The variable path beam is reflected from the surface of the beamsplitter and strikes a mirror 42 attached to the specimen table 10. Thelength of this path changes 6 44. The two beams create an interferencepattern on 2. A system for detecting fatigue damage and surface stressin metals comprising: means for supporting a test specimen; meansattached to the support means for measuring displacement of the testspecimen in a plane; means for generating an ultrasonic wave; meansconnected to the generating means for transmitting the ul- A trasonicwave along a surface of the test specimen; a

equal to a half waveis accomplishedby different g to claim Pwherein themeans tress in mtals accordin source of ccherent radi tion;

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3. A system for detecting fatigue damage and surface

1. A method for determining early fatigue damage and surface stress inmetals comprising the steps of: transmitting an ultrasonic wave across asurface to be examined; directing a beam of coherent radiation at apoint on the said surface, through the ultrasonic wave; detecting thereflected coherent radiation; moving the surface in a known manner;measuring the change in amplitude of the reflected radiation, andcomparing the phase relationship of the reflected radiation to theultrasonic wave whereby variations in said amplitude and phase will beindicative of fatigue damage and surface stress.
 2. A system fordetecting fatigue damage and surface stress in metals comprising: meansfor supporting a test specimen; means attached to the support means formeasuring displacement of the test specimen in a plane; means forgenerating an ultrasonic wave; means connected to the generating meansfor transmitting the ultrasonic wave along a surface of the testspecimen; a source of coherent radiation; means for transmitting saidradiation from the source to the surface; means for measuring theradiation reflected from the surface of the test specimen; display meansconnected to said measuring means to indicate the amplitude of saidreflected radiation; phase detecting means connected to said generatingmeans and said measuring means to detect changes in phase of saidultrasonic wave and said reflected radiation, and display means forproviding an indication of said phase changes.
 3. A system for detectingfatigue damage and surface stress in metals according to claim 2 whereinthe means to indicate amplitude is an oscilloscope.
 4. A system fordetecting fatigue damage and surface stress in metals according to claim2 wherein the display means for indicating phase changes is a digitalcounter.
 5. A system for detecting fatigue damage and surface stress inmetals according to claim 2 wherein the means for transmitting theultrasonic wave is a surface wave transducer.